1. Field of the Invention
This invention relates to new and useful improvements in molded thermoplastic thread protectors for the threaded ends of pipe or casing and to methods of manufacturing thereof.
2. Description of the Prior Art
In shipping industrial pipe, conduit and casing which are threaded at the ends, it has been necessary to provide protectors to prevent damage to the threads. At the present time, one of the most commonly used thread protectors for threaded pipe or casing is one which is made of thermoplastic material, cup shaped and internally threaded to fit the threaded ends of the pipe or casing requiring protection. Thread protectors of this type require considerable time for application and have to be screwed on the ends of the pipe which is a time consuming process. As a result, there has been a considerable need for thread protectors which can be applied without the need for screwing onto the threaded ends of the pipe and which will not slip off the threaded ends during shipment and handling.
Halsey U.S. Pat. No. 3,485,271 discloses an internally threaded protector for pipe threads.
Vestal U.S. Pat. No. 3,744,528 discloses a tubular plastic closure member for metal tubes or pipe.
Callicoatte U.S. Pat. No. 4,239,062, discloses a plastic thread protector for threaded pipe having a cup shaped construction with longitudinally extending ribs for gripping the threads to be protected. The use of longitudinally extending ribs does not provide any means for securing the protector on threaded pipe and there is a tendency to slip off.
Curtiss U.S. Pat. No. 2,900,435 discloses a plastic bushing for the ends of electrical conduit which may be hammered into place. The bushing is formed of plastic material and is internally threaded.
In the formation of molded plastic objects having internal threads and lugs and the like, special consideration has to be given to the design of the molds for forming such parts.
Blaustein U.S. Pat. No. 3,150,222 discloses apparatus for molding threaded articles of thermoplastic by the injection molding process. This involves the use of a two part mold in which one of the parts has threaded recesses forming threads in the molded article. The threaded part of the mold has to be withdrawn from the molded article by unscrewing the article therefrom.
Armour U.S. Pat. No. 3,013,308 discloses a method and apparatus for manufacture of threaded plastic parts. The internal portion of the mold is threaded and the part must unscrewed from the threaded portion of the mold.
Zulaf U.S. Pat. No. 3,350,044 discloses cup shaped covers for the ends of electrical conduit having longitudinally extending internal ribs for securing the covers in place.
Fields U.S. Pat. No. 3,247,548 discloses a method and apparatus for molding internally threaded cup shaped parts of thermoplastic by use of a mold having a collapsible inner portion.
Kessler U.S. Pat. No. 3,325,576 discloses a method and apparatus for molding internally threaded parts of plastic having a two part inner mold segment, one part of which is movable to eject the molded threaded piece from the mold.
Alexandris U.S. Pat. No. 3,584,092 discloses a vacuum molding process for drawing a sheet of thermoplastic material into a female mold cavity and having threaded ribs to be formed in the molded product.
In the manufacture of molded parts of thermoplastic materials by the injection molding process, when the product is manufactured with internal ribs or threads, it has been necessary to remove the molded part by an unscrewing operation or to remove the insert portion of the mold by means of collapsing the same. Thus, it has been necessary to use collapsible mold inserts in order to remove the mold insert from the molded part where under cuts or lugs are produced in the molding operation.
There has not been a practical means provided for the molding of internal ribs which extends circumferentially of a molded part without the use of collapsible mold inserts.